1. Field of the Invention
The invention relates to an arrangement for mutually synchronising two streams of optical pulses, comprising an optical phase detector for deriving a phase error signal from said two streams of optical pulses, and a pulse generator for generating one of said streams of optical pulses with a pulse frequency dependent on the phase error signal
The invention is also related to a receiver and a transmission system using such an arrangement.
2. Description of the Related Art
An arrangement according to the preamble is known from the journal article "Ultra-High-Speed PLL Clock Recovery Circuit Based on All-Optical Gain modulation in Travelling-Wave Laser Diode Amplifier" by Kawanishi and Saruwatari in IEEE Journal of Lightwave Technology, Vol. 11, No. 12, December 1993, pp. 2123-2129.
The trend in future optical communication system will go towards increasing bit rates in the range from 10 to 100 Gbit/s, and even above. A function that is frequently needed in such communication systems is the synchronisation of a stream of optical pulses locally generated in a transmitter or a receiver to a reference stream of optical pulses. This function may be needed in a multiplexer in order to synchronise the multiplexing operation to a reference clock, or in a receiver in which a clock signal present in the received optical signal has to be recovered from said received optical signal. Due to the increasing bit rates it becomes more difficult to realize such synchronisation by using electronic components, presently being suitable for bit rates up to 10 Gbit/s.
To avoid the use of high speed electronic components, it has been proposed to use an optical phase detector to determine the phase difference between an input stream of optical pulses, and a stream of optical pulses generated by the pulse generator. In the arrangement according to the above mentioned article the phase comparator is constituted by a polarisation controller, a 2 by 2 coupling element, a laser diode amplifier and an optical filter. In the laser diode amplifier the combination of the two streams of optical pulses causes a beat signal having a component with a frequency being equal to the difference of the pulse frequencies of both optical pulse streams. This beat signal is caused by the non-linear behaviour of the laser diode amplifier. The optical filter eliminates the optical pulse stream generated by the pulse generator from the output of the laser diode amplifier in order to enhance the beat signal. The amplification properties of the laser diode amplifier will be in general polarisation dependent. To erasure a correct operation of the optical phase detector it is required that the polarisation state of the input signal is adjusted to coincide with the polarisation of the optical pulse stream generated by the pulse generator. Therefor the polarisation controller is introduced into the arrangement.